The idea that I have for the plot of my story revolves around recovering magical texts and artifacts from ancient ruins. My magic system is based around speaking a written language of magic to cast spells and anyone can do so through study and practice and all magic stems from that ancient civilization. The question that I have is what kind of disaster is required for that plot to happen and how would it cause the ancients to leave behind that kind of power and knowledge?
- Something infected the source of magic - causing it to poison anyone who tried to speak it or cast a spell. Eventually the taint dissipated, but not before all the mage's died off and the knowledge was lost
- Ye olde classic; Massive war between two mage factions - caused collapse of society
- Comet/Meteorite impact that wiped out the capital and caused the society to collapse into infighting. Could be a magical or mundane cause
- Attack from magical/ inter-dimensional invaders that wiped the ancient's out. They could return at any time...
- Waning/Waxing of magic - the magic stopped working, which caused the ancient society to collapse and be buried, now it's resurfacing and all that old knowledge is useful again. Could have been a natural or deliberate cause
Without more details on your world/magic system, the cause could be almost anything. You could even leave it as in in-universe mystery and source of great anxiety for your characters.
Some more ideas:
- Whilst magically powerful, wizards still can be attacked physically. The Knight Templars were a quite powerful order but Philip IV of France owed them vast amounts of money. Instead of paying back he gave out sealed orders to his guards and soldiers and ordered them to be opened at a certain date and time. The orders stated that all Knight Templars were to be arrested immidiatly. Because of the sudden and simultanious execution of this order the Knight Templars were not able to resist. Although some smaller chapters of the order survived for some time this lead to the downfall of the order. Something like this could happen to your wizards too. They somehow angered a ruler and he ordered to arrest and/or kill all wizards at once.
- Infighting between the wizards leads to a war were many wizards are killed and much destruction is done. In order to avoid this in the future only very few, selected individuals are teached in the ways of your magic system. Then it just takes a plague, a natural disaster or maybe just even the progress of time to kill the remaining wizards.
- The wizards grow to a very big and bureaucratic organization. But magic is not static and therefore over the course of centuries the magic words shift their meaning and spells don't work like before or at all. The wizard's organization is to bulky to react to this change and looses all of its power. Given the right allocation of resources they could have comprehended the shifts and adapted their spells accordingly which is exactly what your characters would discover
I hope this helps